The IP Multimedia Subsystem (IMS) adopts a Packet Switched access mode, while a Circuit Switched (CS) network adopts a CS access mode. To integrate the IMS and the CS network effectively, IMS Centralized Services (ICS) are proposed. The ICS support the use of CS access to bear media (streams) to implement IMS services.
The Service Centralization and Continuity (SCC) Application Server (AS) (SCC-AS) is an IMS AS that is based on a home network and can support the ICS. The SCC-AS can provide a function of enabling the ICS and act as the agent of a User Equipment (UE) to control an IMS session in an IMS domain.
Currently, an ICS protocol (or known as an I1 protocol) is being formulated, which helps to implement the ICS on a standard and normalized basis. The I1 protocol is an application protocol on an I1 interface, and can support functions similar to the main functions of the Session Initiation Protocol (SIP) (for example, initiation and modification of an IMS session). The I1 interface is located between the UE and the SCC-AS and used to transmit IMS service control signaling.
The architecture of a system based on the I1 protocol is shown in FIG. 1. As shown in FIG. 1, the system includes a CS network 110, an SCC-AS 120, and a UE 130, where the UE 130 is interconnected with the SCC-AS 120 via the CS network 110.
Therefore, it is urgent to solve the problem how to reliably send I1 protocol messages exchanged between the SCC-AS and the UE to ensure effective control over IMS services that are based on CS access.